The present invention relates to devices and methods to measure bulk unconfined yield strength of powders using minimal material. The traditional method of measuring the bulk strength of a powder is to place material in some form of shearing device that allows a consolidation load to be placed on top of the material. The material is then sheared to bring it to a steady shear load condition measured by load cells. The consolidation load is decreased, and the sample is sheared again to measure the peak shear stress. This procedure is typically repeated five to seven times at different shear loads, and all of this data is used to compute a single measurement of unconfined yield strength. The shear cells utilized to accomplish this task hold between 75 cc (cubic centimeters) and 400 cc material and may require multiple fillings. This prior art procedure is complex and requires a trained technician to perform these tests. These devices generally produce a value that is repeatable to only about 30 percent. Often sufficient material is not available for a detailed analysis using these standard methods. This is especially true in the formulation stage of the product or process design.
The inability to characterize the bulk cohesive strength of powders using prior methods can lead to significant production losses in pharmaceutical, food, cosmetic, pigment, chemical, and other industries that handle bulk powders. Companies often desire to characterize bulk strength prior to process design, but have insufficient quantities of material to accomplish this by standard means. Failure to characterize the bulk cohesive properties leads to process failure and costly redesigns, extending time-to-market and wasting company resources. In addition, product quality is strongly influenced by the degree of cohesion a bulk powder material possesses. The ability to obtain a measurement of cohesive flow properties early in the design process provides valuable information to guide product development.
What is needed are new precise methods and devices to measure bulk strength of powder materials, those new methods and devices should require only minimal quantities of powder material and capable of being carried out by relatively low-skilled persons.